PROJECT SUMMARY Parkinson's disease (PD) is a culmination of aging-related changes, genetic predispositions, and environmental insults that result in accumulation of alpha-synuclein (?-syn)-containing Lewy bodies and degeneration of the nigrostriatal system. Neuroinflammation has been proposed to be involved in PD pathophysiology, supported by observations of inflammatory microglia in post mortem PD brains and longitudinal PET imaging that reveals early and sustained microglial activation in the basal ganglia of PD patients. Not only has increased MHC-II expression been observed, it correlates positively with ?-syn burden. Chronic microglial activation can result in a feed-forward cycle of proinflammatory cytokine secretion, production of reactive oxygen species, and subsequent neuronal damage. Activated microglia also have the potential to convert astrocytes to a toxic A1 phenotype that can induce the death of neurons. Both microglia and neurons can signal to peripheral T-cells to invade from the periphery, providing another source of toxic damage to neurons. However, whether neuroinflammation contributes to, or is simply a consequence of nigrostriatal degeneration, remains unclear. Neurotoxicant, ?-syn transgenic and ?-syn vector models of PD have been used to investigate the role of neuroinflammation in nigrostriatal degeneration. Similar to the PD brain, these models reveal neuroinflammation associated with either ?-syn inclusions or nigral degeneration. However, none of these models exhibit the complete sequence of events associated with nigrostriatal pathology in PD: prolonged ?- syn accumulation resembling Lewy bodies followed by protracted degeneration. Recently, our lab has characterized a new rat PD model that recapitulates these features, a model in which nigrostriatal synucleinopathy is induced by intrastriatal injection of preformed ?-syn fibrils (?-syn PFFS). We have observed that peak reactive microglial morphology is evident in the SN at 2 months, corresponding to the time point in which the greatest number of phosphorylated ?-syn (pSyn) inclusions are observed. Further, we observe a pSyn-triggered increase in MHC-II immunoreactive microglia at this same 2-month time point that significantly correlates with pSyn inclusion load. Importantly, we observe reactive microglia and increased microglial MHC-II expression in association with peak load of SNc pSyn inclusions months prior to degeneration, suggesting that neuroinflammation may!contribute to nigrostriatal degeneration. In the present exploratory R21 application we propose to extend these observations regarding the response of microglia to ?-syn inclusions to understanding the role that astrocytes may play in this degenerative cascade. We also will directly test the involvement of microglia in degeneration. The overarching hypothesis of this proposal is that microglial activation, triggered by accumulation of pathological ?-syn, contributes to nigral degeneration.